It sounds like you need to make a swayback adjustment. There are a number of techniques for it, but my personal favorite is simply taking the back piece and, instead of cutting it on a fold, drawing a curve so that it tapers into the narrowest point of your back and then flares out again. This is something I would suggest doing a muslin for before cutting into your final fabric; it always takes me at least a second try to get it right, and the first time I did it, it took several attempts. But it can really improve the fit of a garment--which is the point, isn't it?